1. Technical Field
The present disclosure relates to a power amplification apparatus and a control method for amplifying a transmission signal using multiple class D power amplifiers.
2. Description of the Related Art
A power amplification apparatus incorporated into a transmitter used for wireless communication typically increases the small amplitude of an input signal in order to gain a signal output strength necessary for a radio system and outputs the amplified signal. For amplifying the small amplitude of the input signal to obtain a signal of high output power, the power amplification apparatus consumes much power. Power consumption largely affects the operating time of a radio which is powered by a battery, such as a cellular phone. The power amplification apparatus is accordingly required to have high power efficiency.
One possible means to increase the power efficiency is use of a class D power amplifier. The class D power amplifier is an amplifier that makes use of the saturated operation of transistors and can achieve high power efficiency because, ideally, a current flows during a switching period and no unnecessary current flows.
For a modulation scheme, orthogonal frequency-division multiplexing (hereinafter denoted as OFDM), used in wireless LANs, has been employed in recent years in order to improve spectrum efficiency. The OFDM scheme modulates and multiplexes multiple carrier waves that have different frequencies and are orthogonal to each other. The OFDM scheme has a high power peak relative to the average power at a timing when the phases of the carrier waves overlap. The ratio of the average power to the peak power is represented by peak average power ratio (hereinafter denoted as PAPR), which is about 10 dB in the OFDM scheme, for example. The OFDM scheme inherently has a high PAPR and requires a linear amplifier for suppressing the effect of intersymbolic interference caused by distortion, for example. When the peak power is the saturation power of the power amplifier, the average power assumes a small value relative to the saturation power. In such a situation, the power efficiency at the time of output of the average power is low because the power amplifier cannot be operated at an operating point of high power efficiency. Here, the difference between the maximum power and the average power is called the amount of backoff. As the amount of backoff increases, the power amplifier operates at an operating point with a lower power efficiency.
A switched-capacitor power amplifier is a possible means to improve the power efficiency. A switched-capacitor power amplifier can control the output voltage so as to be linear by preparing multiple small-sized amplifier cells and controlling the number of active amplifier cells with a digital code. An example of the switched-capacitor power amplifier is described in A Switched-Capacitor RF Power Amplifier Solid-State Circuits, IEEE Journal of Volume: 46, Issue: 12 pp. 2977-2987, December 2011. A switched-capacitor power amplifier can be considered as a radio frequency-digital analog converter (RF-DAC) that outputs a high frequency signal having an amplitude controlled by a digital code, improvement of the linearity being the issue to be dealt with. In the description that follows, such a digital code for controlling the amplitude will be referred to as AM code.
A switched-capacitor power amplifier controls the gate voltages of an Nch transistor and a Pch transistor with a carrier wave signal so that the Nch transistor and the Pch transistor alternately turn on to amplify the amplitude of the input signal. In the process, the waveform of the carrier wave signal used for controlling the gate voltages can become rounded (deformed) due to the effect of parasitic resistance or parasitic capacitance of wiring. The Nch transistor and the Pch transistor then would turn on and an unnecessary through current would flow from the Pch transistor to the Nch transistor, causing a reduction in power efficiency. In a conventional practice, this is addressed by changing the duty ratio of the carrier wave signal so as to prevent both the Nch transistor and Pch transistor from turning on at same period due to rounding (deformation) of the waveform of the carrier wave signal.
With a traditional switched-capacitor power amplifier, when the duty ratio is varied as three different values Dα, Dβ, and Dγ (0<Dα<Dβ<Dγ≦0.5) in accordance with output voltage Vout as shown in FIG. 1 for example, a higher output voltage is produced as the AM code becomes larger. In FIG. 2, periods in which the voltage Vout is not output (the ranges ΔVa and ΔVb in FIG. 2) occur when the duty ratio is changed. Thus, a conventional switched-capacitor power amplifier causes degradation in linearity.
To overcome this problem, the technique disclosed in Japanese Unexamined Patent Application Publication No. 2008-172511 improves the linearity by compensating distortion using multiple distortion compensation tables.